1. Field of the Invention
This invention relates to novel crib members for use in building cribbing to support roofs in underground mines opened to remove mineral deposits such as coal, tabular mineral deposits, and metal ore. The crib members of this invention may also be used advantageously in tunnel construction and in all other subterranean excavations where roof and ground control is a matter of concern. In addition, the crib members of the invention may be advantageously used to stabilize soil embankments and to construct jetties and offshore artificial reefs.
2. Description of the Prior Art
Wood has long been the material of choice for crib construction due to its availability and its moderate strength in compression and because, in small dimensions, it is easy to handle. Wood, however, has the disadvantages of relatively low compressive strength and Young's modulus, non-uniformity from piece to piece, flammability, rot and deterioration.
Recently, with the need for more reliable roof support, crib members constructed of ordinary concrete, steel wire reinforced concrete, and steel fiber reinforced concrete are being used in underground mining. Such crib members may have dimensions like ordinary wood cribbing, with a rectangular cross section and lengths up to 48 inches. Alternatively, the concrete members may be constructed as flat disks, typically three inches thick and 22 inches in diameter or as flat washers, typically 22 inches in diameter with a 12 inch diameter perforation, or as concrete cylinders. Such concrete disks, washers or cylinders are stacked vertically in axial alignment to the desired height.
Concrete cribbing has the advantage of greater compressive strength, but tends to fail catastrophically, rather than deforming like wood. The concrete members also tend to require wood spacers to retard horizontal movement, which increases material and installation costs. These drawbacks thus affect both safety and economics.
An example of attempts to improve concrete member cribbing is described in Chlumecky U.S. Pat. No. 4,497,597. This patent describes the use of axially stacked concrete washer members, reinforced by an annular metal band firmly attached to the periphery of the washer. Deformation is into the central hole, since the metal band does not yield radially. A commercial embodiment said to be manufactured under this patent is described in Commercial Shearing, Inc., Catalog M-12 3 MSR 5/87, which shows washer and disc configurations used with thick fiber shims to distribute the load throughout the crib.
Bureau of Mines Report of Investigations 8804, 1983, Smelser & Hanton describes different steel fiber reinforced concrete crib designs for mine roof support using rectangular shapes in conjunction with wood wedging and blocking.
Bureau of Mines Report of Investigations 9161, 1988, Barczak & Schwernmer describes the effect of rate of loading on deformation of wood cribbing for mine roof support.
Bureau of Mines Report of Investigations 9168, 1988, Barczak & Tasillo describes how height, configuration and horizontal displacement of wood cribbing affects its strength and stability.
Bureau of Mines Information Circular 9217, 1989, Barczak, Schwernmer & Tasillo describes factors in designing wood and concrete cribs as gate road supports in longwall mining. Rectangular and flat washer shaped concrete cribbing is disclosed.